The biochemistry of the integration of the DNA of a virus into the chromosome of its bacterial host has been studied. The proteins that carry out this naturally-occurring genetic recombination have been purified and characterized. When acting alone, each protein component is without permanent effect on DNA; recombination is produced only by the combined action of all the components. The component encoded by the bacterial host consists of at least two proteins, each of which is of low molecular weight and is associated with the bacterial chromosome. These proteins include elements responsible for the specific use of supertwisted DNA in integrative recombination. The component encoded by the virus is a protein that functions as a DNA sequence specific element for recombination. The DNA sequences on the viral chromosome that are involved in the specific recognition of this protein have been mapped. Using the techniques developed in the study of integrative recombination, a second naturally-occurring site-specific recombination has been achieved in cell-free extracts.